Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over packet-switched communications networks and, more specifically, to methods and apparatus for providing holdover when the primary timing reference source loses its traceability to UTC.
Description of the Problem and Related Prior Art
Packet-based timing methods are becoming essential for delivering timing over packet-switched networks, often referred to as the cloud. In particular, Precision Timing Protocol (PIP) (aka IEEE 1588-2008) is becoming popular for delivering timing information (time/phase/frequency) from a Grand Master (GM) clock to slave clocks in end application-specific equipment. For example, wireless base stations providing mobile telephony services require precise timing and the backhaul method of choice is Ethernet.
The Grand Master clock provides timing information over the packet-switched network to the slave clocks by exchanging packets with embedded time-stamps related to the time-of-arrival and time-of-departure of the timing packets. The slave clock utilizes this information to align its time (and frequency) with the Grand master. The Grand Master is provided an external reference to serve as the basis for time and frequency. Most commonly this reference is derived from a Global Navigation Satellite System (GNSS) such as the GPS System that in turn is controlled by the US Department of Defense and its timing controlled very precisely and linked to the US Naval Observatory. Time alignment to the GPS clock is, for all practical purposes equivalent to time alignment to UTC.
The Grand Master clock is equipped with a high stability oscillator, typically an ovenized quartz oscillator (OCXO) or a Rubidium atomic standard. The intent is to allow the clock to go into holdover mode and bridge intervals of time when the GPS system is unavailable. That is, if the GPS becomes unavailable, the GM can utilize the local oscillator to “keep time”. The ability to maintain a specified accuracy is directly linked to the quality of the local oscillator.
When a GM loses its reference and goes into holdover mode the system of PTP slave clocks that are synchronizing themselves with the said GM may choose to establish an alternate GM to which they will switch to. However, this involves a significant time delay and in the duration the slave clocks could drift to the extent that the application, such as the mobile telephony system, may experience unacceptable outage.